The present invention generally relates to a method for selectively chlorinating pyridine or a substituted pyridine, and more particularly to such a method conducted in the gas phase in the presence of chlorine as the chlorinating agent.
Chlorinated pyridine derivatives have been prepared by a variety of techniques. One general approach to preparing chlorinated pyridines involves the chlorination of pyridine or of a pyridine derivative with molecular chlorine used as the chlorinating agent (Cl.sub.2, hereinafter referred to as "chlorine"). A number of differing techniques for chlorinating pyridine or its derivatives with chlorine have been developed.
In one facet, thermally-induced chlorinations have been conducted in the vapor phase in a reactor maintained at a generally uniformly high temperature, generally above 250.degree. C. and even 300.degree. to 400.degree. C., as has generally been described in U.S. Pat. Nos. 2,820,791 and 3,153,044. These methods have in the past presented various disadvantages such as poor selectivity and substantial formation of tars that clog the reactor or associated pipes, making a continuous operation difficult.
Another chlorination route involves reactions that are initiated by means of light or ultra-violet rays. Such methods have been described, for example, in U.S. Pat. Nos. 3,297,556 and 4,054,499. These methods, although they can be operated at temperatures lower than thermally-induced methods, have lead to the deleterious formation of tarry by-products that foul the light tubes and give rise to a corresponding reduction in the initiation radiation, and hence in the efficiency of the overall reaction or process.
Still other chlorination routes involve the use of chemical compounds that initiate the chlorination reaction, either in liquid or in vapor phase. While providing enhanced selectivities, particularly in the gas phase, these routes nevertheless require the use of the chemical initiators to achieve acceptable process results.
In light of this background, there remains a need and demand for processes for the chlorination of pyridine or substituted pyridines which provide high selectivities and acceptable yields and conversions. The present invention addresses this need.